In a variety of consumer electronics, solid state drives (SSDs) incorporating non-volatile memories (NVMs) are frequently replacing or supplementing conventional rotating hard disk drives for mass storage. These non-volatile memories may include one or more flash memory devices, such as NAND flash memories, and the flash memory devices may be logically divided into blocks with each of the blocks further logically divided into addressable pages. These addressable pages may be any of a variety of sizes (e.g., 512 Bytes, 1 Kilobytes, 2 Kilobytes, 4 Kilobytes), which may or may not match the logical block address sizes used by a host computing device.
When cells or pages in a block are read from an NVM, the process of reading a page may have a detrimental effect on other pages in the block. The method used to read flash memory can cause surrounding pages in the same memory block to change over time (i.e., become programmed). The read page does not fail, but instead the surrounding pages may fail on a subsequent read. This phenomenon is known as the Read Disturb effect. Read Disturb induced errors typically occur when a wordline (WL) of a NVM is read substantially more frequently than other wordlines on the same block. In particular, the immediately adjacent wordlines are the most prone to errors and show large numbers of errors after enough Read Disturb activity has occurred. The Read Disturb errors can cause possible data loss if the errors are too numerous to correct with error-correcting codes (ECCs).
In order to mitigate Read Disturb errors, known methods track the issue directly by keeping page read counts for every single page. Thus, when a certain threshold for page counts of a particular page is reached, the data of the block can then be relocated in the NVM, and then the original block released to the block pool. The threshold number of reads is generally in the hundreds of thousands of reads between intervening erase operations. More complex solutions may include using sets of criteria to determine when relocate the data, which usually involve the relationship of adjacent page read counts.
Further ways of mitigating Read Disturb include consolidating the data from pages to blocks to reduce the memory footprint. By putting a read threshold on the block, rather than on each page, the block can be conservatively relocated earlier than the failing point. Yet another method is to examine every host read's flash location and then read the surrounding pages to check those pages for higher bit errors. When higher error ratios of surrounding pages occur, the block will then be relocated.